In recent years, in the field of on-vehicle engines, particularly on-vehicle diesel engines, a multistage (two-stage) exhaust turbo-charging system has been adopted in which a high-pressure stage turbocharger having a high-pressure turbine drivable by exhaust gas from the exhaust manifold of the engine cylinder and a low-pressure stage turbocharger having a low-pressure turbine drivable by exhaust gas from the high-pressure turbine are arranged sequentially in the flow path of the exhaust gas, and supply air for the engine pressurized by a low-pressure compressor of the low-pressure stage turbocharger is supplied via an air supply channel to a high pressure compressor of the high-pressure stage turbocharger to be further pressurized by the high-pressure compressor and charged to the engine.
With an engine equipped with a multistage exhaust turbocharger as mentioned above, stable operation of the engine with high turbo-charging efficiency is attained, by increasing super charge pressure in a low and middle speed operating range of the engine by performing two-stage supercharging by allowing both the high-pressure stage and low-pressure stage turbocharger to operate, and by performing single-stage supercharging in a high speed operating range of the engine by allowing only the low-pressure stage turbocharger to operate by allowing the exhaust gas and supply air to bypass the high-pressure stage turbocharger.
FIG. 9 is a schematic representation of such a two-stage supercharging system as disclosed for example in US 2003/0159442 A1, U.S. Pat. No. 6,378,308 B1, and JP 59-82526.
In FIG. 9, reference numeral 103 is an exhaust manifold, 1 is a high-pressure stage turbocharger having a high-pressure turbine 1a and a high-pressure compressor 1b connected to the turbine 1a by a shaft, 2 is a low-pressure stage turbocharger having a low-pressure turbine 2a and a low-pressure compressor 2b connected to the turbine 2a by a shaft.
Reference numeral 117 is an exhaust bypass valve device for controlling the flow rate of exhaust gas flowing in an exhaust bypass pipe 116, 5 is a compressor bypass valve device for controlling the flow rate of bypassed supply air flowing in a high-pressure compressor bypass pipe 111.
When allowing the engine equipped with the two-stage turbocharger to operate in a two-stage supercharging state by allowing both the high pressure stage and low-pressure stage turbocharger 1 and 2, both the exhaust bypass valve device 117 and compressor bypass valve device 5 are closed.
In this state, the exhaust gas flowing out from the exhaust manifold 103 drives both the high-pressure turbine 1a of the high-pressure stage turbocharger 1 and low-pressure turbine 1b of the low-pressure stage turbocharger 2. On the other hand, supply air pressurized by the low-pressure compressor 2b connected to the low-pressure turbine 2a is further pressurized by the high-pressure compressor 1b connected to the high-pressure turbine 1a, and supplied to each cylinder of the engine as charge air for combustion in the cylinders.
Supercharge pressure can be increased in order to increase charging pressure and output of engine by performing two-stage supercharging like this in a low, middle speed operating range of the engine.
When allowing the engine to operate in a single-stage supercharging state by the low-pressure stage turbocharger 2 in a high-speed operating range by allowing the exhaust gas and supply air to bypass the high-pressure stage turbocharger 1, the compressor bypass valve device 5 is opened and the opening of the exhaust bypass valve device 117 is controlled so that desired pressure of charge air is produced.
Most of the exhaust gas flowing out from the exhaust manifold 103 bypasses the high-pressure turbine 1a and joins with a small portion of the exhaust gas flowed through the high-pressure turbine 1a to drive the low-pressure turbine 2a. 
On the other hand, most of the supply air pressurized by the low-pressure compressor 2b connected to the low-pressure turbine 2a bypasses the high-pressure compressor 1b via the high-pressure compressor bypass pipe 111 and joins a small portion of the supply air flowed through the high-pressure compressor 1b to be supplied to each cylinder of the engine as charge air.
In a high speed operating range of the engine, stable operation of the engine with high turbo-charging efficiency is realized by performing single-stage supercharging by the low-pressure stage turbocharger 2 by allowing most of the exhaust gas and supply air to bypass the high-pressure stage turbocharger 1.
A comparatively large installation space is required in the engine room to install an engine equipped with a two-stage exhaust turbocharger of this type because it is inevitably increased in bulk as compared with a single-stage one.
It is required in the case of on-vehicle engine to reduce the bulk of the two-stage exhaust turbocharger as far as possible in order to mount an engine equipped with a two-stage exhaust turbocharger in the conventionally narrow engine room of the vehicle.
However, in engines equipped with a two-stage exhaust turbocharger disclosed in U.S. Pat. No. 6,378,308 B1 and JP 59-82526, particularly an air supply path from a low-pressure compressor 2b to a high-pressure compressor 1b and an air supply bypass path from the low pressure compressor 2b to a compressor bypass valve device 5 on a high-pressure compressor bypass pipe 111 are arranged in a horizontal plane, so the size in the horizontal direction of the two-stage exhaust turbocharger is large, and it is difficult to secure an installation space for mounting the engine in a narrow engine room of a vehicle.
As to the engine equipped with a two-stage exhaust turbocharger disclosed in US 2003/0159442 A1, a high-pressure stage turbocharger 1 and low-pressure stage exhaust turbocharger 2 are arranged three dimensionally in order to make it possible to connect them with exhaust and air supply paths of minimum length and reduce the bulk of the two-stage exhaust turbocharger assembly. However, as the casings of the high-pressure stage and low-pressure stage turbochargers are composed completely separately with exhaust and air supply channels for connecting them being provided, the bulk of the two-stage exhaust turbocharger assembly is still not reduced enough, and it is still not easy to secure an installation space for mounting the engine in a narrow engine room of a vehicle.